1. Field of the Invention
The present invention is directed to a plasma system and method and, in particular, to an annular electrode discharge, non-thermal plasma device for the pre-treatment of combustion air and method for operating the same.
2. Description of Related Art
Studies investigating the augmentation of flames with plasma date back over several decades. The benefits of pre-treating the air used for combustion are well known, namely flames can burn with a leaner fuel-air mix and at lower temperatures resulting in increased fuel efficiency and reduced emissions.
A combustion process is generally initiated and maintained by heating the bulk gas to a temperature (typically in the order of approximately 1000° C.) where free radicals such as O, OH, H are formed that can initiate dissociation and oxidation reactions. In the case of pure hydrocarbons, complete molecular conversion will result in the formation of carbon dioxide and water which can be released directly to the atmosphere. The chemical efficiency of the molecular conversion depends on the generation and propagation of free radicals, which effectively break carbon bonds. The efficiency of production and the concentration of these radicals depend primarily on the temperature of the combustion gas. To achieve the high temperature that is necessary for such conversion, considerable enthalpy must be added to the gaseous medium. Sufficiently high temperatures can be realized by direct heating, for example, using an incinerator or a thermal electric arc. Recently, it has been shown that an alternative and efficient way to produce radicals and promote combustion may be in electrical discharges that generate non-thermal plasmas, as disclosed in the publication by Penetrante B. M., and Schultheis S. E., entitled “Non-Thermal Plasma Techniques for Pollution Control”, NATO ASI Series G, Vol. 34, Parts A and B (1992).
Plasma is a term used to denote a region of ionized gases. Plasma can be created by bulk heating of the ambient gas (as in a flame) or by selectively energizing electrons by electrical energy (as in electrical discharges). Non-Thermal Plasmas (NPT) are ionized gases which are far from local thermodynamic equilibrium (LTE) and are characterized by having electron mean energies significantly higher than those of ambient gas molecules. In NTP, it is possible to preferentially direct the electrical energy in order to produce highly energetic electrons with minimal, if any, heating of the ambient gas. Instead, the energy is almost entirely utilized to directly excite, dissociate and ionize the gas via electron impact.
The idea of using an electrical discharge to increase the net enthalpy extracted from chemical combustion and reduce regulated emissions is one that has been investigated for decades, as disclosed by Chen, D. C. C., Lawton, J., and Weinberg, F. J., in the publication Tenth Symposium on Combustion, pp. 743–754 (1965). Chen and his colleagues discussed the approaches tried and problems encountered by he and other scientific investigators of his day. The idea was to create the free radical precursors necessary to the combustions process by electrically “heating” the appropriate molecules rather than thermally heating the bulk fuel-air mixture. The problems encountered then, and still remain today, are centered on the following two factors: i) the uniform creation of free radicals, ii) without expending more energy than that extracted from the chemical reaction.
Early attempts to use an arc discharge to enhance the combustion process presented several problems. The arc discharge manifests itself as a narrow high temperature filament. These filaments are only 10 microns in diameter thus treating a limited amount of the reactants. The treated reactants are heated to temperatures so high that the energy generated by the chemical reaction is miniscule compared to the electrical energy deposited. Attempts to “spread out this” energy to the whole of the fuel air mixture by enhancing the diffusion process, i.e., by introducing turbulence, or by moving the whole arc filament around bodily, e.g., via an applied magnetic field, proved either ineffective or impractical.
The type of plasma that is needed to enhance combustion must be non-thermal, operate at substantially atmospheric pressure, and be generated substantially uniformly over a relatively large volume. Non-Thermal Plasma can produce energetic electrons, typically in the range of approximately 1 eV −10 eV, which effectively leads to the creation of free radicals without adding to the bulk gas the enthalpy necessary to reach very high temperatures as recognized by Penetrante et al., in the publication entitled “Non-Thermal Plasma Techniques for Abatement of Volatile Organic Compounds and Nitrogen Oxides”, INP Report XIII; B. Müller, Ed., pp. 18–46 (1996) and in the book by Tarnovsky V. and Becker K., Plasma Sources Science and Technology, 4, 307 (1995).
It is therefore desirable to put these studies to practical use by developing an efficient and practical self-stabilizing discharge electrode to generate non-thermal plasma for the pre-treatment of combustion air.